Many portable devices and electronic systems require a stable power supply. Devices may be sensitive to variations in the power-supply voltage, so providing a stable power-supply voltage is critical. However, the device may draw varying amounts of current from the power supply as internal transistors are switched on and off. Such current variations make providing a constant power-supply voltage difficult. When large amounts of current are suddenly sunk by the device, the power-supply voltage can drop to dangerously low levels that may cause storage upsets or logical failures.
A monolithic power converter chip or block in a larger system may have a large power transistor may be used to drive current through an inductor. The inductor smoothes changes in the large current provided by the power transistor, and additional filter capacitors store charge to help stabilize the power-supply voltage. However, the power transistor needs to be turned on and off to restore charge on the filter capacitors and thus maintain the desired power-supply voltage.
The power-supply voltage may be sensed and compared to a target voltage. However, measuring the current through the power transistor or inductor can provide more rapid feedback and better control of the power-supply current. Having multiple sources of feedback, such as both the power-supply voltage and the power current can provide extremely fast response times, such as responses in micro-seconds rather than just milli-seconds.
What is desired is a power-current sensor and control circuit. A power control circuit is desired that can quickly sense power current and adjust a power transistor to maintain a stable power supply.